The possibility was investigated that class I MHC antigen can serve both as restriction elements and as nominal antigens. It was found that T cell hybrids can be generated which are self-Ia restricted but which are specific for class I MHC alloantigens, suggesting that they recognize a composite determinant composed of class I and class II MHC molecules. T cell recognition of such composite determinants in intigens can be seen as nomial antigens in association with self-Ia restriction determinants.